c++: Implement C++26 P2786R13 - Trivial Relocatability [PR119064]HEADtrunkmaster

The following patch implements the compiler side of the C++26 paper.
Based on the https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119064#c3
feedback, the patch enables the new conditional keywords
trivially_relocatable_if_eligible and replaceable_if_eligible only
for C++26, for older versions those conditional keywords yield
-Wc++26-compat warning and are treated as normal identifiers.
Plus __trivially_relocatable_if_eligible and __replaceable_if_eligible
are handled as conditional keywords always without diagnostics (similarly
to __final in C++98).
The patch uses __builtin_ prefix on the new traits (but unlike clang
which for some weird reason chose to name one __builtin_is_replaceable
and another __builtin_is_cpp_trivially_relocatable this one uses
__builtin_is_replaceable and __builtin_is_trivially_relocatable.
I'll try to convince clang to change, they've only implemented it
recently.
The patch computes these properties on demand, only when something needs
them (at the expense of eating 2 more bits per lang_type, but I've recently
saved 64 bits and a patch to save another 64 bits is pending; and even
4 bits wouldn't fit).
The patch doesn't add __builtin_trivially_relocate builtin that clang has,
std::trivially_relocate is not constexpr and I think we don't need it for
now at least until we implement some kind of vtable pointer signing
__builtin_memmove should do the job.  Especially if libstdc++ will for clang
compatibility use the builtin if available and __builtin_memmove otherwise,
we can switch any time.
I've cross-tested all testcases also against the clang++ trunk
implementation, and both compilers agreed in everything except for
https://github.com/llvm/llvm-project/issues/143599
where clang++ was changed already and
https://github.com/llvm/llvm-project/issues/144232
where I believe clang++ got it wrong too.
The first testcase comes from
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2786r13.html#simple-worked-examples
just tweaked so that the classes are named differently each time and that it
compiles.  There are 3 differences from the paper vs. the g++ as well as
clang++ implementation, I've added comments into
trivially-relocatable1.C but I think either that part of the paper wasn't
updated through the later changes or it got it wrong.

2025-07-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/119064
gcc/
	* doc/invoke.texi (Wc++26-compat): Document.
gcc/c-family/
	* c.opt (Wc++26-compat): New option.
	* c.opt.urls: Regenerate.
	* c-opts.cc (c_common_post_options): Clear warn_cxx26_compat for
	C++26 or later.
	* c-cppbuiltin.cc (c_cpp_builtins): For C++26 predefine
	__cpp_trivial_relocatability=202502L.
gcc/cp/
	* cp-tree.h: Implement C++26 P2786R13 - Trivial Relocatability.
	(struct lang_type): Add trivially_relocatable,
	trivially_relocatable_computed, replaceable and replaceable_computed
	bitfields.  Change width of dummy from 2 to 30.
	(CLASSTYPE_TRIVIALLY_RELOCATABLE_BIT,
	CLASSTYPE_TRIVIALLY_RELOCATABLE_COMPUTED, CLASSTYPE_REPLACEABLE_BIT,
	CLASSTYPE_REPLACEABLE_COMPUTED): Define.
	(enum virt_specifier): Add VIRT_SPEC_TRIVIALLY_RELOCATABLE_IF_ELIGIBLE
	and VIRT_SPEC_REPLACEABLE_IF_ELIGIBLE enumerators.
	(trivially_relocatable_type_p, replaceable_type_p): Declare.
	* cp-trait.def (IS_NOTHROW_RELOCATABLE, IS_REPLACEABLE,
	IS_TRIVIALLY_RELOCATABLE): New traits.
	* parser.cc (cp_parser_class_property_specifier_seq_opt): Handle
	trivially_relocatable_if_eligible,
	__trivially_relocatable_if_eligible, replaceable_if_eligible and
	__replaceable_if_eligible.
	(cp_parser_class_head): Set CLASSTYPE_REPLACEABLE_BIT
	and/or CLASSTYPE_TRIVIALLY_RELOCATABLE_BIT if corresponding
	conditional keywords were parsed and assert corresponding *_COMPUTED
	macro is false.
	* pt.cc (instantiate_class_template): Copy over also
	CLASSTYPE_TRIVIALLY_RELOCATABLE_{BIT,COMPUTED} and
	CLASSTYPE_REPLACEABLE_{BIT,COMPUTED} bits.
	* semantics.cc (referenceable_type_p): Move definition earlier.
	(trait_expr_value): Handle CPTK_IS_NOTHROW_RELOCATABLE,
	CPTK_IS_REPLACEABLE and CPTK_IS_TRIVIALLY_RELOCATABLE.
	(finish_trait_expr): Likewise.
	* tree.cc (default_movable_type_p): New function.
	(union_with_no_declared_special_member_fns): Likewise.
	(trivially_relocatable_type_p): Likewise.
	(replaceable_type_p): Likewise.
	* constraint.cc (diagnose_trait_expr): Handle
	CPTK_IS_NOTHROW_RELOCATABLE, CPTK_IS_REPLACEABLE and
	CPTK_IS_TRIVIALLY_RELOCATABLE.
gcc/testsuite/
	* g++.dg/cpp26/feat-cxx26.C: Add test for
	__cpp_trivial_relocatability.
	* g++.dg/cpp26/trivially-relocatable1.C: New test.
	* g++.dg/cpp26/trivially-relocatable2.C: New test.
	* g++.dg/cpp26/trivially-relocatable3.C: New test.
	* g++.dg/cpp26/trivially-relocatable4.C: New test.
	* g++.dg/cpp26/trivially-relocatable5.C: New test.
	* g++.dg/cpp26/trivially-relocatable6.C: New test.
	* g++.dg/cpp26/trivially-relocatable7.C: New test.
	* g++.dg/cpp26/trivially-relocatable8.C: New test.
	* g++.dg/cpp26/trivially-relocatable9.C: New test.
	* g++.dg/cpp26/trivially-relocatable10.C: New test.
	* g++.dg/cpp26/trivially-relocatable11.C: New test.

0 files changed, 0 insertions, 0 deletions